With the realization that end of scaling for conventional complementary metal-oxide-semiconductor (CMOS) integrated circuits is fast approaching in the semiconductor industry, alternative nanostructures and materials have been investigated. Of such nanostructures and materials, carbon nanotubes (CNTs) offer excellent intrinsic properties that are suitable for high performance nanoscale devices.
A key advantage of CNTs over conventional CMOS devices is that scaling limitations of metal-oxide-semiconductor field effect transistors (MOSFETs) due to boundary scattering of electrons from imperfect interfaces are solved naturally in CNTs which have a smooth, well coordinated graphene structure with no bonds to the outside. This enables CNTs to retain excellent transport properties to much smaller lateral dimensions than silicon. The small radius and possibility of completely surrounding the CNT by a gate provide excellent electrostatic confinement of channel electrons, enabling the channel length to be scaled down to very small dimensions, and their small size would enable high packing densities. Band structure calculations of CNTs according to P. Avouris and J. Chen, “Nanotube electronics and optoelectronics,” Materials Today, 9, 46 (2006) show that conduction and valence bands are minor images of each other, i.e., both electrons and holes should share equally good transport properties. This indicates suitability of CNTs for a general-purpose high-performance complementary circuit technology.
As is now well known, CNTs can be either metallic or semimetallic, depending on their chirality and have a bandgap which is inversely proportional to their diameter for the semiconducting tubes. The idealized electron/hole dispersion relation is hyperbolic in shape, with a quasi parabolic “effective mass” regime at lower energies and a linear “constant velocity” regime at higher energies, where the limiting velocity, vlim, is ˜5-10×107 cm/sec according to G. Pennington and N. Goldsman, “Semiclassical transport and phonon scattering of electrons in semiconducting carbon nanotubes,” Phys. Rev. B 68, 045426 (2003).
Transport properties are further enhanced by the weak coupling of the charge carriers to acoustic phonons and the fact that the optical phonons have large energies of ˜0.15 eV. All of these factors lead to extraordinarily large mobilities, reported at ˜105 cm2/V-sec at room temperature by Perebeinos et al., “Electron-Phonon Interaction and Transport in Semiconducting Carbon Nanotubes,” Phys Rev. Lett. 94, 086802 (2005).
The intrinsic properties of CNTs make them good candidates for ballistic transport, and several signatures for ballistic transport have been found, for example, in Javey et al., “High-Field Quasiballistic Transport in Short Carbon Nanotubes,” Phys. Rev. Lett., 92, 106804 (2004). The excellent ballistic transport properties of CNTs may be surpassed only by the ballistic transport properties of electrons in vacuum tubes.
According to U.S. Pat. No. 2,242,275 to Varian, a vacuum tube Klystron as an ultra-high frequency amplifier is disclosed. The '275 patent is incorporated herein by reference to illustrate the operating principle of a vacuum tube Klystron.
Referring to FIG. 1, a schematic diagram demonstrating the operating principles of a vacuum tube Klystron is shown. The vacuum tube Klystron consists of an electron stream within a vacuum enclosure (not shown), confined by a magnetic filed to a tight beam. Electrons are emitted from the cathode by thermionic emission and are accelerated by the positive potential V0 at the anode. A series (two or more) of microwave cavities are placed along the electron beam so that the microwave electric field modulates the velocity of the electron stream and causes bunching of the electrons. This results in a strongly amplified current and is the basis for the success of the vacuum tube Klystron as an ultra-high frequency amplifier and generator with extensive use in radar applications.
Obviously, vacuum tube Klystrons are bulky vacuum tube devices that cannot be easily integrated with solid state devices in a circuit despite their excellent ultra-high frequency amplification characteristics.
Therefore, there exists a need for a solid state ultra-high frequency amplifier device that utilizes ballistic transport property of a semiconductor material and methods of operating the same.